Performance degradation and failure mechanisms of pultruded carbon/glass-fiber hybrid sucker rods after long-term downhole service

Pultruded carbon/glass-fiber hybrid composite rods are increasingly used as sucker rods, yet the relationship between degradation accumulated during long-term downhole service and subsequent longitudinal splitting during retrieval and drum winding remains insufficiently understood, particularly for rods containing a carbon/glass hybrid transition region. This study compared five specimens sectioned from a pristine 22 mm-diameter parent rod with five specimens sectioned from a service-aged parent rod retrieved after 435 days of actual downhole service. Tensile, compressive, flexural, and apparent horizontal shear tests were combined with Welch's t-tests, bootstrap-assisted two-parameter Weibull analysis, scanning electron microscopy, dynamic mechanical analysis, and moisture-desorption measurements to characterize mechanical degradation and matrix/interfacial damage. After service, the mean tensile, flexural, and apparent short-beam shear strengths decreased by 10.02%, 19.92%, and 21.98%, respectively. After Holm correction, the reductions in flexural and apparent short-beam shear strength remained statistically significant, whereas the tensile-strength reduction did not (adjusted p = 0.057). The mean compressive strength decreased by only 2.25% and showed no statistically significant difference; however, its coefficient of variation increased from 2.42% to 18.88%, while the Weibull shape-parameter estimate decreased from 41.31 to 5.27, suggesting greater compressive-strength dispersion. For the carbon-fiber core, the storage modulus decreased by 34.3%, whereas the glass-transition temperature increased from 159.5 to 164.76 °C. These changes may reflect competing contributions from residual post-curing and moisture-related effects, although the underlying chemical mechanisms were not directly characterized. The greater equilibrium mass loss of the service-aged specimens during drying was consistent with greater retention of removable moisture. Scanning electron microscopy observations revealed extensive matrix fragmentation, resin loss, fiber-matrix debonding, and longitudinal interfacial cracking. Collectively, the results support a possible two-phase damage-evolution process in which prolonged fluid exposure and repeated operational loading progressively weaken the matrix and hybrid interfaces, while additional bending during retrieval and drum winding may promote the propagation and coalescence of pre-existing cracks, thereby contributing to local longitudinal splitting. These findings provide insight into the relationships among field exposure, loading-mode-dependent degradation, strength dispersion, and interfacial damage in three-region hybrid composite sucker rods.